(1) Field of the Invention
The present invention relates to a pneumatic tire, and more particularly, the invention relates to an improvement on a so-called all season type pneumatic tire which is used throughout the year from the winter season to the summer season, and is to provide the tire of this type which is favorably applied to the heavy duty pneumatic radial tires particularly for use in trucks, buses and so on.
(2) Description of the Prior Art
It is well known that as to the tires which are frequently used throughout the year, the performance required for the tires in the winter season during which they are obliged to run on ice and snow roads differ from those in the case of the other seasons. That is, while the first priority is imposed upon the high performance on ice and snow during the winter, particularly a low rolling resistance (low fuel consumption), a high wear resistance and so forth are needed during seasons other than the winter.
Heretofore, with respect to the so-called all season type tires which are intended to be used throughout the year, on the assumption that they begin use in the winter, there have been proposed such tires in which the tread pattern is composed of a so-called block pattern consisting of independent blocks from the new tire stage to the tire wearing middle stage, and the pattern is changed to a so-called rib pattern having ribs extending in the circumferential direction subsequently to the middle wear stage, whereby the performance required in all seasons are met. In such tires, in order to obtain higher performance on ice and snow, it is common practice to use as a tread rubber a rubber of a peculiar composition which gives a relatively large frictional force particularly at a low temperature. However, the tires in which rubber of this kind is used as the tread rubber have the defects that since they generally have a large rolling resistance and a poor wear resistance, they are inversely disadvantageous during the seasons other than the winter and especially when in use on good roads.